Cells from Y-1 mouse adrenocortical tumor are maintained in tissue culture and are injected into mice to produce tumors. The action of ACTH upon the steroidogenic activity of the cells will be examined by incubating mitochondria with cholesterol and measuring the production of pregnenolone (side-chain cleavage). The influence of cycloheximide upon the side-chain cleavage system will be investigated by measuring transport of cholesterol-3H into mitochondria from cells incubated with and without ACTH. The role of microfilaments and microtubules in the response to ACTH will be investigated by studying the possible influence of agents such as cytochalasin, vinblastine and colchicine upon the response to ACTH. Finally the capacity of host mice to enhance steroidogenic activity when tumor cells are injected following repeated culture in vitro, will be studied by investigating the effect of mouse plasma upon th cells in vitro and of such procedures as hypophysectomy, adrenalectomy and thyroidectomy on the capacity of the host mice to increase the steroidogenic activity of the injected cells.